1. Field of the Invention
The present invention relates to a method for transmitting data from multiple wireless tire condition sensors, and more particularly to a collision avoidance method for transmitting data from multiple wireless tire condition sensors.
2. Description of the Related Art
Tires are one of the most important items on vehicles. The condition of tires is critical to driving safety. To facilitate drivers' constant awareness of tire pressure, most new vehicles come with wireless tire pressure monitoring system. The wireless pressure monitoring system has multiple wireless tire condition sensors respectively mounted in all tires of a vehicle, wirelessly transmits acquired tire pressure information to a wireless tire pressure receiver inside the vehicle on a periodic or random basis, and provides tire pressure information to the driver at all time after the tire pressure information is analyzed and displayed by the wireless tire pressure receiver for the driver to quickly respond to an abnormal tire pressure condition.
With reference to FIG. 3, a conventional wireless tire pressure monitoring system has at least four wireless tire condition sensors 71˜74 and a wireless tire pressure receiver 70. Each wireless tire condition sensor 71˜74 has a control unit, a sensing element, a wireless high-frequency transmission circuit, a wireless low-frequency reception circuit and a power supply. Each wireless tire condition sensor 71˜74 has an exclusive 8-bit code or code with higher bits before delivery.
When activated, each tire pressure sensor 71˜74 starts using the wireless high-frequency transmission circuit to periodically or randomly transmit sensed tire pressure information and its exclusive code encoded specifically to the wireless tire pressure receiver 70. After receiving and decoding the encoded information, the wireless tire pressure receiver 70 first checks the exclusive code to identify if the received tire condition information pertains to in-vehicle information or comes from other vehicles. If the latter is the case, the received tire condition information is discarded to avoid further processing of the tire condition information of other vehicles. If the former is the case, the wireless tire pressure receiver 70 determines from which tire the received tire condition information comes and further analyzes tire pressure, temperature and other relevant parameters contained in the tire condition information.
As the single wireless tire pressure receiver 70 serves to receive tire condition information from all the wireless tire condition sensors 71˜74 alone, a feasible method is to separate the data transmission time of all the wireless tire condition sensors 71˜74 in a vehicle. Although theoretically feasible, if the data transmission cycle of each wireless tire condition sensors 71˜74 is fixed, the data transmission cycles of some wireless tire condition sensors 71˜74 eventually overlap after the wireless tire condition sensors 71˜74 continue transmitting data for a period of time. Data collision thus occurs in the wireless tire pressure receiver 70 when the data transmission cycles of the wireless tire condition sensors 71˜74 overlap.
To solve the overlap of the data transmission cycles of the wireless tire condition sensors 71˜74, another method is to increase the frequency of data transmission in the data transmission cycles of each wireless tire condition sensor 71˜74. Conventional wireless tire condition sensors 71˜74 actually transmit tire condition information multiple times in a data transmission cycle. If the frequency of transmitting tire condition information in each data transmission cycle increases, the possibility of data collision can be relatively reduced and the tire condition information transmitted by the tire pressure sensors 71˜74 can all be successfully received by the wireless tire pressure receiver 70.
Similar technique is disclosed in U.S. Pat. No. 6,486,773, entitled “Method for communicating data in a remote tire pressure monitoring system”, wherein each wireless tire condition sensor 71˜74 transmits tire condition information multiple times in a data transmission cycle, and a wake-up time for the wireless pressure sensor 71˜74 to wait each time when data transmission of the tire condition information occurs can be preset with a time duration differing from those for the rest of wireless pressure sensors 71˜74 to wait at the same time spot when data transmission of tire condition information of the rest of wireless pressure sensors 71˜74 occurs. Suppose that the data transmission cycle is 42 microsecond (μs) and total number of data transmission in each data transmission cycle is eight, one of the wireless tire condition sensors 71 is preset with a wake-up time sequence represented by (6, 8, 6, 6, 4, 4, 4, 4), another wireless tire condition sensor 72 is preset with a different wake-up time sequence represented by (4, 4, 6, 8, 6, 6, 4, 4), and the wake-up time is measured in microseconds. In a data transmission cycle, the wireless tire condition sensor 71 first waits 6 μs before transmitting tire condition information for the first time, then waits 8 μs before transmitting the tire condition information for the second time and waits the wake-up time in the remaining wake-up time sequence (6, 6, 4, 4, 4, 4) before transmitting the tire condition information for the rest of six more times respectively. Similarly, the wireless tire condition sensor 72 waits to transmit tire condition information thereof eight times according to the wake-up time sequence (4, 4, 6, 8, 6, 6, 4, 4). Given the method, each wireless tire condition sensor 71˜74 can transmit same tire condition information multiple times in each data transmission cycle to ensure that the wireless tire pressure receiver 70 does not lose tire condition information transmitted from any one of the wireless tire condition sensors 71˜74 due to data collision among the wireless tire condition sensors 71˜74.
Taiwanese Patent Publication No. 200736079 or U.S. Pat. No. 7,269,530, entitled “Method for wirelessly transmitting sensed signals of tires” also discloses that each wireless tire condition sensor 71˜74 transmits same tire condition information multiple times in a data transmission cycle. A wake-up time sequence is similarly set up for same tire condition information of each wireless tire condition sensor 71˜74 transmitted multiple times in the data transmission cycle to ensure that the wireless tire pressure receiver 70 does not lose tire condition information transmitted from any one of the wireless tire condition sensors 71˜74 due to data collision among the wireless tire condition sensors 71˜74. The method differs from the foregoing method in further taking acceleration of vehicle into account to calculate the time spent for each turn of a tire and allocating the time for each turn to a number of data transmission for tire condition information. For instance, suppose that each turn of the tire takes 90 μs and the number of data transmission for tire condition information N=3, the time allocated to a wake-up time for transmitting tire condition information once is 30 μs, that is, each wireless tire condition sensor 71˜74 sequentially transmits same tire condition information thrice with the same wake-up time in each turn of the tire.
However, the drawbacks of the foregoing methods lie in that as each wireless tire condition sensor 71˜74 is hermetically mounted in a corresponding tire, and battery replacement for the wireless tire condition sensors 71˜74 is not as convenient as that for regular electronic equipment. Hence, the wireless tire condition sensors 71˜74 must be efficient in terms of power management. Higher frequency of data transmission in each data transmission cycle gives rise to more power consumption contradicting the principle of power management for the wireless tire condition sensors 71˜74.
U.S. Pat. No. 6,931,923, entitled “Tire condition monitoring apparatus” discloses another technique having a transmitter-receiver 40 and four transponders 30. The transmitter-receiver 40 is mounted inside a vehicle and the four transponders 30 are respectively mounted in four tires. The transmitter-receiver 40 transmits interrogating radio waves to the transponder 30 in each tire for the transponder 30 to reply with tire condition data, such as tire pressure and the like.
The transmitter-receiver 40 wirelessly transmits an inquiry multiple times to each transponder 30 within a first time duration so that the transponder 30 within each tire can reply with sensed tire condition information. The first time duration is determined in accordance with a speed of the vehicle, and a number of the inquiry transmitted within the first time duration varies with the vehicle speed. For instance, when the vehicle speed is slower than 100 km, the number of the inquiry is once per minute, and when the vehicle speed is faster than 100 km but equal to or lower than 200 km, the number of the inquiry is twice per minute.
To ensure that the transmitter-receiver 40 can receive tire condition information transmitted by each transponder 30, a relative angle between an antenna of the transmitter-receiver 40 and the transponder 30 mounted on a rim of a wheel is further considered so as to determine a number of the transponder 30 transmitting and receiving signals in a turn of the tire. The tire condition monitoring apparatus determines the number with totally five equations as follows.
Equation 1 serves to calculate the revolution per second R(20) of the tire when the vehicle speed is 20 km per hour.
Equation 2 serves to calculate the time spent for each revolution of the tire when the vehicle speed is 20 km per hour.
Equation 3 serves to calculate the time Tp of the transponder 30 located within an optimal detection angle (relative to an antenna 41) in each revolution of the tire when the vehicle speed is 20 km per hour.
Equation 4 serves to calculate the time Tk of the transponder 30 located within the optimal detection angle (relative to an antenna 41) after deducting the time spent for each inquiry of the transmitter-receiver 40.
Equation 5 serves to calculate the number of inquiry between the transmitter-receiver 40 and each transponder 30 using Tp and Tk within each revolution of the tire when the vehicle speed is 20 km per hour.
To correctly receive tire condition information transmitted from each transponder 30, the tire condition monitoring apparatus employs many equations to calculate the number of inquiry for tire condition information within each revolution at a specific vehicle speed. As a result of many equations involved, the conventional tire condition monitoring apparatus are more complicated technically and the cost thereof is relatively higher.